The present invention relates to an autofrettage process and, more particularly, relates to the autofrettage of thick-walled cylinders through the cyclical application of high pressure to the internal bore of the cylinder.
In the design of thick-walled cylinders which are utilized as compressor cylinders, pump cylinders, high pressure tubing, process reactors and vessels, nuclear reactors, isostatic vessels, or gun barrels and the like, various fatigue and fracture characteristics must be taken into consideration when selecting the material and in choosing the various dimensions. Cylinders of this type which are generally subjected to sudden and frequently dramatic pressure fluctuations, such as the pressure generated in a gun barrel upon the firing of the weapon, pressure reversals in pump cylinders or in process reactors employing high-pressure piping, are prone to premature failure as the result of material fatigue or embrittlement produced by the internal pressures to which the bores of the cylinders are exposed. Usually, the material employed for such cylinders is steel; normally an alloy steel having superior strength and fatigue properties.
In order to alleviate the problems encountered in subjecting thick-walled cylinders to alternating or fluctuating, internally acting high pressures, there has been developed a process for enhancing the strength of thick-walled cylinders which are subjected to repeated internal pressures. This process is commonly referred to as "autofrettage" and involves the application of such interior pressure to the bore of the cylinder so as to plastically deform at least the inner layers of the cylinder material beyond the elastic limit or yield strength of the material and to thereby generate "negative" or residual tangential compressive stresses at the cylinder bore. These residual stresses will counteract the destructive effects of the internal cyclical or intermittent high operating pressures to which the cylinder is subjected so as to appreciably extend the service life of the cylinder. This plastic deformation of the inner bore of the cylinder beyond the elastic limit of the material so as to increase the service life is a function of the strength of the material, in essence, the type of steel or alloy utilized, and of the ratio of the outer diameter to the inner diameter of the cylinder. In effect, the basic advantage obtained through the autofrettaging of thick-walled cylinders which are subjected to cyclical, intermittent or fluctuating internal high operating pressures lies in that the maximum stresses of the cylinders in response to these operating pressures are decreased to a considerable extent to thereby impart a longer service life to the cylinders.